refactor: move key commands under auth group and fix help consistency

Related to https://gitlab.com/postgres-ai/platform/-/issues/346

## Summary
- Reorganize CLI by moving `show-key` and `remove-key` commands under the `auth` command group
- Convert `auth` from a standalone command to a command group with `login` as the default subcommand
- Remove deprecated `add-key` command (use `auth --set-key` instead)
- Fix inconsistent help text capitalization ("Prepare" → "prepare")

## Changes

**Before:**

| Command | Description |
|---------|-------------|
| `auth` | authenticate via browser (OAuth) or store API key directly |
| `add-key` | store API key |
| `show-key` | show API key (masked) |
| `remove-key` | remove API key |

**After:**

| Command | Description |
|---------|-------------|
| `auth` | authentication and API key management |
| `auth login` | authenticate via browser (OAuth) or store API key directly |
| `auth show-key` | show API key (masked) |
| `auth remove-key` | remove API key |

## Usage
- `pgai auth` — runs OAuth login flow (default)
- `pgai auth --set-key <key>` — store API key directly
- `pgai auth show-key` — show masked API key
- `pgai auth remove-key` — remove API key

Author: claude

cli/bin/postgres-ai.ts

@@ -204,7 +204,7 @@ program
 
 program
   .command("prepare-db [conn]")
-  .description("Prepare database for monitoring: create monitoring user, required view(s), and grant permissions (idempotent)")
+  .description("prepare database for monitoring: create monitoring user, required view(s), and grant permissions (idempotent)")
   .option("--db-url <url>", "PostgreSQL connection URL (admin) to run the setup against (deprecated; pass it as positional arg)")
   .option("-h, --host <host>", "PostgreSQL host (psql-like)")
   .option("-p, --port <port>", "PostgreSQL port (psql-like)")
@@ -1534,8 +1534,10 @@ targets
   });
 
 // Authentication and API key management
-program
-  .command("auth")
+const auth = program.command("auth").description("authentication and API key management");
+
+auth
+  .command("login", { isDefault: true })
   .description("authenticate via browser (OAuth) or store API key directly")
   .option("--set-key <key>", "store API key directly without OAuth flow")
   .option("--port <port>", "local callback server port (default: random)", parseInt)
@@ -1779,16 +1781,7 @@ program
     }
   });
 
-program
-  .command("add-key <apiKey>")
-  .description("store API key (deprecated: use 'auth --set-key' instead)")
-  .action(async (apiKey: string) => {
-    console.warn("Warning: 'add-key' is deprecated. Use 'auth --set-key <key>' instead.\n");
-    config.writeConfig({ apiKey });
-    console.log(`API key saved to ${config.getConfigPath()}`);
-  });
-
-program
+auth
   .command("show-key")
   .description("show API key (masked)")
   .action(async () => {
@@ -1806,7 +1799,7 @@ program
     console.log(`Config location: ${config.getConfigPath()}`);
   });
 
-program
+auth
   .command("remove-key")
   .description("remove API key")
   .action(async () => {